The present invention relates to a film carrier semiconductor device and, more particularly, to a film carrier semiconductor device suitable for high-density packaging.
A film carrier semiconductor device has a package of a type called a tape automated bonding type and can be made thinner and smaller than packages of the other types. Therefore, film carrier semiconductor devices are widely used in, e.g., general consumer watches and calculators.
FIGS. 8A and 8B are plan and sectional views, respectively, showing the main part of a conventional film carrier semiconductor device of this type. This film carrier semiconductor device comprises an elongated film carrier tape 1 consisting of a polyimide film or the like, a semiconductor chip 2 with bumps 7 as metal projecting portions formed on electrode pads, leads 4 for external connections, which are formed into a desired pattern by etching a metal foil, such as a copper foil, adhered to the surface of the carrier tape 1, and pads 5 for electrical selection, i.e., for performing electrical selection or a bias test by selectively bringing a probe of a meter into contact with the pads 5 in the form of the film carrier tape. The film carrier tape 1 has carrying and positioning sprocket holes 6 and a device hole 3 for mounting the semiconductor chip 2. Note that reference numeral 8 denotes a suspender for supporting the leads 4.
To fabricate the film carrier semiconductor device having the above arrangement, the film carrier tape 1, the leads 4, and the bumps 7 formed on the semiconductor chip 2 are bonded together by thermocompression bonding or eutectic bonding, i.e., by inner lead bonding (ILB). Subsequently, with the semiconductor chip 2 mounted on the film carrier tape 1, a probe is brought into contact with each pad 5 for electrical selection to perform electrical selection, a bias test, or the like, thereby completing the film carrier semiconductor device. In general, the suspender 8 as an insulating film frame is formed on the film carrier tape 1 to prevent deformation of the leads 4, or resin encapsulation is performed by potting a resin 9 in order to improve the reliability and obtain mechanical protection.
To mount the film carrier semiconductor device as described above on a printed circuit board, as shown in FIG. 8C, the leads 4 are cut into a desired length to separate the semiconductor chip 2 from the film carrier tape. Subsequently, the semiconductor chip 2 is fixed on a printed circuit board 11 by an adhesive 10. Thereafter, the leads 4 are bonded to bonding pads 12 on the printed circuit board 11 by outer lead bonding (OLB), thereby completing packaging of the semiconductor chip 2.
In this film carrier semiconductor device, since bonding between the semiconductor chip 2 and the leads 4 for external connections can be performed at once regardless of the number of leads, the speed of the bonding process is high. In addition, the use of the film carrier tape 1 facilitates automatization of assembly, such as bonding, and tests, such as electrical selection. Therefore, the film carrier semiconductor device has advantages such as good mass production properties.
Furthermore, packaging can be performed at higher densities than in ceramic packages or plastic packages, and it is also possible to realize a small size, a light weight, and a low profile.
One currently available semiconductor chip which is lightest and thinnest, and can be packaged at a highest density is a flip chip. As is well known, the flip chip is fabricated by forming lattice-like electrodes on the surface of a semiconductor chip, forming bumps consisting of solder or the like on these electrodes, and connecting these bumps directly to bonding pads on a printed circuit board. This flip chip is the most advanced form in terms of a light weight, a low profile, and a small size. The flip chip is also effective and appropriate in increasing the number of terminals because electrodes can be arranged on the entire surface of a semiconductor chip.
Although, however, the flip chip has a number of advantages as described above, a large difference is present between the thermal expansion coefficient of silicon as the main material of a semiconductor chip and that of a printed circuit board, and this produces a stress against connecting portions. Since the connection strength is degraded by this stress and it is very difficult to perform electrical screening at a high temperature, i.e., a burn-in test before packaging, the range of applications of the flip chip is greatly limited.
The film carrier semiconductor device, on the other hand, can keep its high connection reliability by the buffering effect of the leads 4 even after mounted on a printed circuit board and also has an advantage that the burn-in test can be easily performed using the pads 5 for electrical selection. However, the film carrier semiconductor is inferior to the flip chip in packaging density by the amount of an area where the leads 4 are present.
Also, the conventional film carrier semiconductor device described above comprises a film carrier tape consisting of a polyimide resin and leads consisting of a copper foil, and the thermal expansion coefficient of the polyimide resin is about 2.0.times.10.sup.-5 .degree. C..sup.-1 and that of Cu is about 1.7.times.10.sup.-5 .degree. C..sup.-1, whereas the thermal expansion coefficient of the semiconductor chip consisting of Si is about 2.5.times.10.sup.-6 .degree. C..sup.-1 ; that is, a large difference is present between their thermal expansion coefficients. Because of this large difference, when the film carrier semiconductor device is subjected to a thermal shock test, such as a temperature cycle test, a repetitive stress is applied to the leads to finally disconnect them since the expansion and contraction of the film carrier tape are larger than those of the semiconductor chip.
To prevent this, the thermal expansion coefficient of the polyimide resin may be decreased. For example, when film carrier semiconductor devices using, as film carrier tapes, Kapton having a thermal expansion coefficient of 2.0.times.10.sup.-5 .degree. C..sup.-1, available from DU PONT-TORAY CO., LTD. and Upilex S having that of 1.5.times.10.sup.-5 .degree. C..sup.-1, available from Ube Industries Ltd., were subjected to a temperature cycle test at a temperature of -65.degree. C. to 150.degree. C., leads of the device using Kapton were disconnected after about 100 cycles, while those of the device using Upilex S were not disconnected even after about 300 cycles. Therefore, if the thermal expansion coefficient of the film carrier tape is further decreased to be equivalent to that of the semiconductor chip, the service life of the device may be prolonged. In this case, however, the thermal expansion difference from Cu constituting the leads is increased. Therefore, when, for example, a Cu foil to be used as leads is laminated during the fabrication process of the film carrier tape, the film carrier tape is largely curled due to the thermal expansion difference to make it difficult to maintain the flatness of the film carrier tape.
In general, the Cu foil is laminated on the film carrier tape via an adhesive. In this case, since lamination is performed by heating the adhesive, the temperature is high during lamination and then decreased to room temperature after that. Therefore, a curl is produced in the laminated film carrier tape due to the difference between thermal expansion coefficients. In general, therefore, a film carrier tape and a Cu foil having substantially equal thermal expansion coefficients are often used, and to meet the temperature cycle, a lead length in a device hole is increased to absorb the stress by a lead portion. In this case, however, the degree of lead deformation is increased.